Retroviruses can produce disease in humans and animals through a wide variety of mechanisms. Our long term goal is to describe in detail the early events in retroviral replication that are required to establish stable and productive infection of the host cell, with a focus on how viral DNA is able to access host DNA and become integrated functionally into the host cell genome. We are using the avian sarcoma virus (ASV) as a model system for in vivo and in vitro experiments;comparative studies with human immunodeficiency virus are included in our research plan. The results of our proposed research will provide a more accurate and complete description of how retroviruses interact with host cells. The findings will be critical for the development of new anti-viral therapies and improvement of retroviral vectors for gene therapy. Our proposed research on retroviruses includes studies of DNA repair, and epigenetic regulation, processes that are highly relevant to mechanisms of cancer induction, progression and treatment. Our proposal is designed to test several hypotheses regarding virus-host interactions. The Specific Aims are: 1) Determine the integration site preferences for ASV. We will test the hypothesis that chromatin structure and species- specific factors can influence integration site-selection. We will use genomic methods in vivo, and chromatin integration substrates in vitro. 2) Test the hypothesis that position-independent mechanisms contribute to proviral epigenetic silencing. We will analyze epigenetic regulation of ASV at defined chromosomal positions and use an siRNA-based high throughput functional screen to identify host factors that regulate position-independent epigenetic silencing of integrated DNA. 3) Investigate the role of host factors in the early steps of ASV infection. We will identify host factors that mediate nuclear import of the ASV pre-integration complex and repair of the integration site using in vivo and in vitro methods. We will also initiate an siRNA-based high throughput functional screen to identify new host factors that mediate these steps. The knowledge obtained from our studies will increase our understanding of retroviral replication and also provide new insight into cellular functions of relevance to normal cell growth and oncogenesis.